Posted and Free-Flow Speeds for Rural Multilane Highways in Georgia

The National Highway System Designation Act of 1995 repealed the national maximum speed limit. As a result, approximately 70% of the United States has increased posted speed limits to 104.7 km/h (65 mi/h) for select multilane highways. This study evaluates the application of the Highway Capacity Manual multilane highway rules-of-thumb for free-flow speed to both an 88.6 and a 104.7 km/h (55 and 65 mi/h) posted speed limit condition. The paper further quantifies the observed relationship between the posted speed limit and observed free-flow speed on rural multilane highways in Georgia. Specific issues evaluated include heavy vehicle influence, traffic volumes, access point density, and vertical grade. The research indicates that the current Highway Capacity Manual rule-of-thumb free-flow estimation technique based on posted speed limit does not adequately estimate free-flow speed for the higher speed limit condition.